1. Field of the Invention
This invention relates to an improved nanotube switch design wherein the nanotube structure is not required to stretch in only one dimension when alternating between off and on positions.
2. Description of the Related Art
Integrated circuit switches such as utilized in the formation of memory cells are constructed in a number of ways giving rise to a number of different switches or memory cells used for formation of many different types of memory, including read only memory (ROM), programmable read only memory (PROM), electrically programmable read only memory (EPROM), dynamic random access memory (DRAM), and static random access memory (SRAM). Important characteristics of these and other memory cells include low cost, programmability (ability to write to), erasability, nonvolatility, high density, low power, and high speed.
Furthermore, the electrical characteristics of the materials used in constructing such switches, such as electromigration of the materials and low resistance, are important as well. Electromigration in metal interconnects and metal-filled vias is one of the key causes of failures in integrated circuit structures.
Actual metallic interconnects that exhibit low resistivity (i.e., a bulk resistivity ˜10−6 Ω-cm at room temperature, such as copper) also contain a large number of crystallographic and morphological defects, such as impurities, dislocations, vacancies, rough surface morphology, and grain boundaries, as well as a thickness usually close to the mean free path of electrons. Excessive electron-ion scatterings also normally occur at these defects so that higher resistivity, higher rate of Joule heating, and more ion transport results. Failures are hence usually initiated at the site of these defects. On the other hand, the physical layout of the interconnects can also generate a distributed density of electron currents inside the conductors and creates preferential failure sites (i.e., corners of via-wire contacts). Due to all these mechanisms, it is virtually impossible to sustain a current density of ˜106 A/cm2 for a long period of time, even when using state-of-the-art copper interconnects.
More recently, carbon nanotube structures have become of interest due to their superior attributes, including low resistivity, high thermal conductivity, and high melting temperature, when, for example, the carbon nanotube material is compared to copper. In Segal et al. U.S. Pat. No. 6,643,165, issued Nov. 4, 2003, it has been proposed to form a carbon nanotube memory cell for an integrated circuit structure using a ribbon or mat of such carbon nanotubes. The description of such a carbon nanotube memory structure found in the aforesaid Segal et al. patent is hereby incorporated herein by reference.
However, a problem with utilizing the flexibility of the carbon nanotube ribbons in proposed structures to form one or more switches having on/off positions is that the forces exerted on the carbon nanotube ribbon to move it from an “off” position to an “on” position are not identical to the forces needed to return the switch from an “on” position back to an “off” position, as would be desirable in a memory cell to assure nonvolatility of the cell.
As seen in the prior art structure of FIG. 1, where the switch is shown in the “off” position, the unstretched carbon nanotube strip or fiber 10 in the middle of memory chamber 20 lies in the same plane as secured end portions 14 and 16 of the carbon nanotube strip, i.e., in an “at rest” or “off” position. In contrast, the prior art structure of FIG. 2 shows carbon nanotube fiber 10 in an “on” position wherein the middle portion of the carbon nanotube is in a stretched position, i.e., in an “in tension” position. The following equations 1 and 2 below illustrate the amount and type of energy needed to respectively move such carbon nanotube switch structures from an “off” position to an “on” position and from an “on” position back to an “off” position.Felectrostatic+Fvan der Waals>Felastic (Switch from “off” to “on”)  Equation 1Felectrostatic+Felastic>Fvan der Waals (Switch from “on” to “off”)  Equation 2
As can be seen in prior art FIG. 1, nanotube strip 10 needs to be elastically stretched and pulled down by the electrostatic force applied to lower electrode 24 to establish a conductive path from lower electrode 24 through the nanotube strip 10 to either first end 14 and/or opposite end 16 of carbon nanotube strip 10 corresponding to the switch's “on” position. To be turned off, the van der Waals forces holding the nanotubes in place must be overcome by pulling the nanotubes toward charged upper electrode 26, such that Equation 2 is satisfied.
It would be desirable to provide a balanced carbon nanotube switching structure which would take approximately the same energy to move the switch from one setting to the other (i.e., from an “off” position to an “on” position, or from an “on” position back to an “off” position); and the same amount of energy to maintain the switch in either the “off” or “on” position once the switch is originally set.